DHT Phono Stage Test

High gain stage with DHT

Some time ago a colleague  (Shawn Fox) contacted me to find out whether I could test some rare high-mu DHTs. I didn’t have them in my stash, so he offered to send them across for testing. He was quite keen to find out the performance with a gyrator load due to the particular characteristics of the DHT in question. The valve in question is the CX-340. There isn’t much information about this valve am afraid and coincidentally, Thomas Mayer (Vinyl Savor) wrote not long ago a review of this valve.

Tracing the curves, the first step

The high anode resistance as well as the low anode current in which this valve operates makes it a real challenge to implement successfully. Hence, this is why the gyrator load plus an output follower stage comes into play as the best companion for this valve. Before we look into the circuit itself, I submitted the 40 valve to the mercy of my tracer:

A very linear triode indeed. The specs between the pair of valves varied quite a bit, this was expected anyway.  Here is the best sample out of the pair:

With a mu of about 30-32 and a transconductance of 200 μmhos,  the anode resistance is about 135-150kΩ. A real challenge for either a resistor load or a choke/transformer. 

A Spice cx340 spice model was easily developed:

The circuit

If you want to use a valve of this type in a RIAA stage, then a circuit like the below is the best option as the gain stage:

I already shown the SiC MOSFET follower stage on this post. The gyrator is well known, so won’t cover it at all.  The filament bias introduces degeneration which helps with improving the linearity of the valve (not needed) and increases the overall impedance presented by the valve. As the stage is a mu-follower, the output impedance is only determined by the J1 FET. This is ideal with a valve of this type. However, due to the low anode current, the output impedance is higher and we also need a follower to avoid slew rate issues and also increased distortion when the stage is loaded. This stage can be easily the 2nd RIAA stage as well as the output as it provides about 30dB of gain.

The DC-coupling of the valve to the follower is ideal. You need the high impedance of the SiC (or MOSFET) gate to improve linearity and increase the bandwidth of the stage, in particular with low current and high-anode resistance like this valve. In my practical implementation, I used a SiC MOSFET, here is the famous AOT1N60, which works just fine as well.

The penalty of increased anode resistance is that we need to increase C1. If you want to avoid this, you can opt for 2 or 3 SiC Schottky diodes (like the C3D02060) to reduce the dynamic impedance presented in the cathode (filament). 

Testing the stage

A new rat’s nest was built for this, and opened the way to other experiments which I hope to publish shortly.  Here is how it looks:

Ignore the bottom Gyrator PCB for now, it is a converted screen regulator for pentode tests. 

So how does it measure?

Really well in my view! You can get 3Hz-35kHz flat response at 30dB. Not bad at all for an DHT. I didn’t measure the miller capacitance, that is something to bear in mind for the RIAA stage driving this valve. 

The distortion as well as the harmonic profile is very nice. The CX-340 provides the expected decay of harmonics of a DHT. I measured distortion to be 0.022% @ 1Vrms output. 

What are the disadvantages? Well, microphonic noise as expected. You get some ringing at 200Hz as well as 2kHz. Probably with proper dampening and care this can be managed. I’m not sure of how it will really perform in a RIAA Stage.  Alternatively, the EML30A should be considered for a similar job.  Pricey but a great valve indeed. There has been an evolution here and Emission Labs has taken care of tackling many of the challenges of a high-mu DHT valve. 

Next time, I looked at alternative Russian directly-heated pentodes to play somehow a similar role and will share my findings.